1. Field of the Invention
The present invention relates to a ball mounting apparatus and ball mounting method for sucking up balls, which are used when an integrated circuit is mounted on a substrate or a substrate-mounted integrated circuit is mounted on other large substrate, for holding the balls on a ball transfer member which is called a head, and mounting the balls on a workpiece.
2. Description of the Related Art
In conventional ball mounting apparatus and methods, various counterbalance mechanisms for heads have been devised to avoid the application of excessive loads to balls by a head when the head mounts the balls onto workpieces such as semiconductor integrated circuits, circuit boards and so on. Such apparatus and methods are described in U.S. Pat. No. 5,768,775 and Japanese Patent Application Laid-Open Hei 9-18129. The apparatus described in U.S. Pat. No. 5,768,775 uses a spring force that cancels a self-weight of the head, but the device has the disadvantage that vibration occurs when the head moves at medium or high speeds because the head is not clamped.
In the apparatus and method described in Hei 9-18129, a head weight is not counterbalanced by a spring tension force, but instead by a lifting force of a cylinder. In the case of counterbalancing the head weight, there are hence two disadvantages with the apparatus and method described above. First, it is difficult to position the head at a predetermined position in a counterbalanced condition. Second, upon moving, a clamping force of the head is weak since the head is fixed only by the self-weight of the head. The first disadvantage of the techniques described in Hei 9-18129 is as follows. If the lifting force of the cylinder is even slightly stronger than a total weight of the head and elements moving with the head (hereafter “head weight”), the head rises to stop at an upper moving limit of the cylinder. On the other hand, if the lifting force of the cylinder is weaker than the head weight, the head drops to stop at a lower moving limit of the cylinder. Only where the lifting force of the cylinder is equal to the head weight, does the head stop in a counterbalanced condition. But, even in the counterbalanced condition, the head moves some distance until a velocity of the head damps due to friction of the cylinder. That is to say, a stop position of the head is not definite. In this manner, the apparatus which uses a cylinder for a balancer has a serious disadvantage. The second disadvantage is as follows. In the case where the head quickly stops when raising or lowering, the head vibrates up and down whether the head weight is heavy or light, because the head is fixed only by the self-weight of the head. The vibration does not cease until the vibration is damped by friction of the cylinder. Only when moving the head in a quasi-static state can vibration be avoided, which is not practical. As mentioned above, a serious disadvantage, in practical use, belongs to an apparatus of which the head is clamped only by the self-weight of the head.